1. Field of the Invention
The present invention generally relates to photomultiplier tubes (PMT) and more particularly to an automatic gain stabilization system used with photomultiplier tubes.
2. Description of the Prior Art
Photomultiplier tubes, hereby shortly referred to as photomultipliers, are common instruments in science and technology for detecting weak light levels. The photomultiplier typically consists of a photosensitive cathode, a chain of secondary emission electrodes called dynodes and an output electrode called an anode with electric potentials arranged between them. The operation principle is as follows:
Light flux hits the cathode which converts light photons into free electrons. The applied voltage directs the free electrons to the first dynode. One electron liberates several secondary electrons from the first dynode in a process called secondary emission. These secondary electrons are in turn directed to the next dynode where the secondary emission process also happens. These steps are repeated on every dynode.
The gain of the photomultiplier is defined as the ratio of the anode current to the cathode current. The gain is typically from 10.sup.2 to 10.sup.8 depending on the number of dynodes, on interdynode voltages and on the dynode materials. The gain should naturally remain stable during the operation in order to yield ideal performance for the light detecting device. Unfortunately this is not usually achieved because the gain tends to drift with the temperature, with variable light fluxes and with ageing of the photomultiplier.
For correcting the gain instabilities a known solution is to employ a supplementary pulsed light source with standardized intensity to monitor the output of the photomultiplier and to adjust the gain according to the obtained signal by, e.g. a feedback loop as presented by Ried and Gilland (U.S. Pat. No. 3,515,878). The pulsed light source can be e.g., a low-intensity lamp, a light emitting diode (LED) or a radioactive isotope in conjunction with an appropriate scintillator.
Another solution is to monitor the first dynode pulse height and adjust an LED amplitude with it as presented by Oikari and Nurmi (U.S. Pat. No. 5,157,250). This solution provides a photomultiplier tube with a gain stabilization system that is insensitive to the drifts encountered with the stabilization light sources. A problem in this solution is that the anode pulse capacitively affects the first dynode. That is why later dynode voltages must be removed during measuring of the first dynode voltage.